Sen. Bernard Sanders promised to live up to his "grumpy grandpa" reputation Monday, giving a gruff rebuke of GOP priorities and also criticizing the president's approach to working with Congress.

The Vermont independent threw flames at both national political parties during a lunchtime appearance at the National Press Club, calling the Democratic Party "out of touch" and repeatedly referring to the GOP as a "right-wing extremist party." But he made no promises to torch Majority Leader Mitch McConnell's plans to return to regular order on appropriations from his perch on the Senate Budget Committee. After eight years in the Senate, the longest-serving independent in congressional history ascended to ranking member of the influential panel, opposite Sen. Michael B. Enzi, R-Wyo., the staunch conservative who holds the gavel.

Sanders, sporting a uncharacteristically tame mane, then ticked off his list of populist budget priorities. He wants to expand Social Security, invest in alternative energy, regulate Wall Street and close corporate tax loopholes. The crowd seemed to eat it up, applauding his attack on "billionaire families" paying to finance major legislation in Washington.

The potential 2016 presidential candidate did not reveal whether he will launch a bid for president, or whether he would run as a Democrat or third-party candidate. He did, however, offer some advice to President Barack Obama on working with Capitol Hill.

Sanders said he respects the president, but he thinks the Democrat made a "major mistake." After organizing a coalition of young, minority and "working class" voters to win the White House in 2008, Sanders said Obama "thought he could sit down with Republicans and negotiate all of these fine agreements — he was mistaken."

The answer shed some light on how Sanders might work with Congress if he were elected chief executive.

Sanders said there was only one way for a president "taking on the billionaire class" to succeed. "The only possible way is to mobilize tens of millions of people to say to Congress, 'Guess what? This is what you are doing.'" He warned, "We are watching you, and if you don't vote for this legislation, you're not going to return to office."

Sanders said gridlock is not the true cause of dysfunction in Congress.

"It's not that every member of Congress has a personality defect and is unable to communicate with the people in the other political party," Sanders said. "The case is right now that the United States Congress is not representative of where the American people are. They're way out of touch."

Sanders spent the first 15 minutes of his hour-long appearance giving the crowd a refresher on his political history — from getting trounced in his early campaigns to founding the Congressional Progressive Caucus as a member of the House — then 17 minutes criticizing the "grotesque and growing level of wealth and income inequality."

The curmudgeon took a softball question in his final minutes on stage. Asked which Ben and Jerry's ice cream flavor he would use his political clout to save from the company's chopping block, Sanders opted for a classic: "Chocolate."

At the end of the speech, two-thirds of the room hopped to their feet to applaud.